Why is it that 3×3 stairs won’t build, but 2×3 + 1×3 will

dwarf-fortress

I have a 3×3 column of up-down stairs that I've dug out, and accidentally broke through to the caverns before I was ready. In trying to block the bottom, I dug out the bottom set of up-down stairs and tried to replace them with an up stairs construction.

DF won't let me build a 3×3 up stair in the space; it says it's blocked. However, it does let me build a 2×3 stair and a 1×3 stair to fill the same space. Why?

z+1 is the level above the build issue where the last up/down stair (designation) is.. z is where the 2 up-stair constructions are (2×3 on top, 1×3 is the row on the bottom) and z-1 is cavern.

z+1      z       z-1
-----    --££-   .--  
-XXX%    £<<<%   -----
%XXX%    £<<<%   ,,',-
%XXX%    £<<<%   '𝓇-'-
%%%%%    ☼☼☼%%   ---.`

Best Answer

After consulting your diagrams, it seems that you are building all of the staircases you want/need to, you just couldn't build them in a batch, correct?

If that's the case, then there are a few one-off issues that could have caused this -- trying to order the construction while the previous structures are still there; transient issues with claimed building materials being or not-being in the appropriate square.

In short; I'd not worry about it, as I don't think this is a common scenario to run into.

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